Color shifting security elements have been used in numerous applications, e.g. as a security feature on banknotes, identity documents, documents of value, documents of rights, tax banderoles, security labels, branded goods and the like, to protect them against counterfeit and illegal reproduction. Color shifting security elements can be produced by applying a coating composition (e.g. an optically variable ink) containing an optically variable pigment (OVP), such as a flake pigment disclosed in e.g. U.S. Pat. No. 4,424,010; U.S. Pat. No. 5,084,351; U.S. Pat. No. 5,171,363; EP 227 423, U.S. Pat. No. 5,653,792; U.S. Pat. No. 5,279,657; U.S. Pat. No. 5,766,738; U.S. Pat. No. 5,571,624; U.S. Pat. No. 5,569,535; U.S. Pat. No. 5,383,995; U.S. Pat. No. 5,278,590; U.S. Pat. No. 5,059,245; U.S. Pat. No. 4,838,648; U.S. Pat. No. 5,214,530 onto an appropriate substrate surface. Printings realized with optically variable inks (OVI®) exhibit the property of changing color upon variation of the angle of observation.
Optically variable pigments are interference pigments which exhibit a substantial angle-dependent color variation, i.e. which exhibit clearly observable different colors under e.g. an orthogonal and a grazing view. This applies for pigments having a low refractive index interference layer (refractive index n smaller than 2.0). Pigments with a high refractive index interference layer (n larger than 2.0), such as TiO2 coated mica pigments, do not exhibit a substantial angle-dependent color variation.
To increase the color gamut of optically variable inks, it can be of advantage, as disclosed in EP-B1-0 227 423, to add soluble dyes or insoluble pigments to the ink formulation, to modify the reflection characteristics of the optically variable pigment and/or to block out unwanted spectral domains in order to achieve predefined color properties. For example, a Cromophtal yellow pigment (from Ciba-Geigy) can be added to a gold-to-green optically variable ink, to block out the reflected blue spectral parts at 400 nm, therefore giving a brighter appearance to the golden orthogonal-view color of the gold-to-green OVI®.
At present, coatings shifting from a first color to a second color, as well as from a first color to black in going from orthogonal to grazing view, are used on currency for its authentication by the unaided eye. Black at grazing view is achieved by mixing the optically variable pigment with a conventional dye or pigment, which has a complementary color to that of the optically variable pigment at grazing view. U.S. Pat. No. 5,059,245 discloses an ink comprising optically variable flakes and a pigment, dispersed in the ink vehicle for blocking out undesired colors at angles of incidence higher than first and second angles of incidence.
Black printing is noteworthy important in currency design, as it allows for a good visibility of the printed features even under difficult light condition, as well as for the easy incorporation of certain types of security elements. For design reasons, there is thus a need for optically variable inks which have black as one of their appearances.
However, the said coating, which shifts from a first color at orthogonal view to black at grazing view, suffers from the drawback that the black at grazing view is difficult to see. Indeed, due to the physics of the system, the black in said coatings only appears in a very narrow range of observation angles (vide infra), and due to physiology and psychology of human vision, certain colors, such as claret, olive or violet, already appear per se almost black at grazing view.